Keywords

Introduction

Gran Canaria is a circular volcanic island dating back more than 14.5 Ma, but eruptions have continued until very recent time. The geological evolution of the Gran Canaria comprises four main volcanic stages, alternating with periods of erosion (Balcells et al. 1992; Carracedo et al. 2002; Ancochea et al. 2004; Guillou et al. 2004; Pérez Torrado 2008; Schmincke and Sumita 2010). The first volcanic stage of the island was built during the Miocene (14.5–7.3 Ma) by a shield volcano of basic composition, which developed later a kilometric caldera collapse in the central part of the island. Two generations of superimposed stratovolcanoes were developed inside the caldera: Tejeda and Roque Nublo volcanoes. The Tejeda Volcano was related to the Miocene activity (Cycle I), but had a salic composition (rhyolitic to trachyphonolitic). The Roque Nublo Volcano was developed during the Pliocene (5.6–2.9 Ma) forming the basic to salic deposits of the Roque Nublo Cycle. The Plio-Quaternary activity of the last two stages is mainly related to fissure eruptions of basaltic composition (Post-Roque Nublo and Recent Cycles). This last activity developed a number of fields of cinder cones and their lavas, which have been related to a NW-SE rifting.

Several intensive erosive stages have been developed between the main volcanic cycles. The most significant flank collapses and large rock slides of Gran Canaria have been triggered in relation to these periods of erosion, in particular, at the final phases of the Miocene shield volcano (~ 9 Ma) and the end of Roque Nublo Cycle (3.5–2.9 Ma) (Funck and Schmincke 1998; Mehl and Schmincke 1999; Yepes et al. 2011). The oldest evidence of flank instability has been related to breccias levels which are found in discordant contact between the lavas of the Miocene shield volcano, located in the southwestern sector of the island (Schmincke 1993; Schmincke and Sumita 2010). Other evidence of this phenomenon would be the coastal morphology in arc extending between La Aldea and Agaete villages, in the western sector of the island (Coello and Coello 1999). This coast defines a deep coastal cliff that has some collapsed rock masses on his foot (Criado et al. 1998) and seems to involve the volcanic materials that define the western border of the Tejeda caldera (Yepes et al. 2011). Our research has been also motivated by the anomalous distribution of lithologies of the different volcanic cycles in the field, in particular the location of the deposits of the Roque Nublo Cycle in the southern and northern sectors of the island. These observations have been considered as evidence of two large flank collapses: one that took place in the Roque Nublo stratovolcano at the end of the Roque Nublo Cycle (García Cacho et al. 1994; Pérez Torrado et al. 1995), and another that would have occurred in the northeastern sector of the island (Hansen 2009). Finally, additional evidences of flank instabilities have been described in the offshore sector of the island. The anomalous topography found in this sector has been interpreted as submarine deposits of debris avalanches and debris flows, which can cover wide areas of the sea floor, related to large flank collapses (Funck and Schmincke 1998; Krastel et al. 2001; Acosta et al. 2003).

This paper provides geological and geomorphologic observations that support the existence of flank instabilities in the northwest sector of the Gran Canaria, in a circular sector of about 175 km2, which could be defined between the Agaete, Arucas and Cruz de Tejeda villages (Fig. 1).

Fig. 1
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Topographic map showing the location of the northwest sector of the Gran Canaria (Canary Islands, Spain). The dashed black square shows the area of Fig. 3.

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The geomorphological observations are focused on the analysis of the drainage system, scarps, erosion surfaces, residual reliefs, cinder cones and offshore debris avalanche deposits. The geologic evidence is derived from the lithological relationships between the surface geology and 71 lithological columns that reach depths between 20 and 460 m (Fig. 4, Table 1). The surface data were derived from geological maps (Balcells et al. 1986; Barrera and Gómez 1986; Balcells and Barrera 1987a, b; Balcells et al. 1992), whereas the underground data were obtained from the wells and boreholes reported in Galindo et al. (2011). The descriptions of the lithological columns (Table 1) were obtained from the database of boreholes developed by the Geological Survey of Spain (Instituto Geológico y Minero de España, IGME) during different research projects.

Table 1 Wells and boreholes data of the NW sector of Gran Canaria Canaria (cf. Galindo et al. 2011)
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Geomorphological Observations

The drainage system is centrifugal and it is modified by sharp river diversions with a NW-SE trend. These diversions seem to be related to the main NW-SE trend of the feeding dykes of the Post-Roque Nublo Cycle, conditioning the differential erosion of the substrate. These small anomalies in the drainage system would be a morphological expression of the NW-SE extension that would have undergone on Gran Canaria during the Post-Roque Nublo Cycle. The streams with the steepest slopes are located on the lateral edges of the study area (Agaete and Guía). At least one embedded stream that describes very sharp changes in its path can be recognized in both margins. The Berrazales (or Agaete) Ravine would be the most typical case in the western sector, and the San Andrés and Moya ravines in the eastern sector. These streams increase sharply their relative divergence close to the populations of Fontanales and Fagagesto, defining a wide circular sector of the island with a relief organization which is uncorrelated laterally with the geomorphology observed in the surrounding areas. These observations suggest that these streams with a variable path have been fitted in favour of very penetrative planes of weakness in the rock mass. These planes could be related to the lateral flanks of a large rock slide. This phenomenon is also observed at a small scale in the area between Agaete and Gáldar, coinciding with a morphostructural scarp (S5). In addition, at the base of this scarp, a number of rock slide deposits have been recognized (Fig. 2).

Fig. 2
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Panoramic view of the NW sector of Gran Canaria between Gáldar and Agaete villages, showing the erosional surfaces (S0, S2, S4 and S6) and the location of the rock slides deposits. The photo has been taken from Montaña Almagro

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In the coastal zone, most of the ravines are interrupted or deviated by a wide flat-bottomed valley with a NW-SE trend and a few isolated cinder cones (Gáldar, Gallinero and Montaña Clavijo) of the Recent Cycle (Fig. 3). The peripheral location of this small volcanic field could be in agreement with lithostatic unloading related to the foot area of a slipped mass. These observations suggest that the occurrence of the flank instabilities in Gran Canaria could be extended during the Post-Roque Nublo Cycle. In addition, the head area is disconnected from the summit of the island by a number of cinder cones of the Recent Cycle (e.g. Pinos de Gáldar, Juncalillo and Montañón Negro). This observation suggests the existence of a lateral spreading due to the intrusion of dykes during the Post-Nublo Roque and Recent cycles, a process that has already been described in other volcanic islands (McGuire et al. 1990; Elsworth and Voight 1995; Voight and Elsworth 1997; Day et al. 1999; Galindo 2005; Cecchi et al. 2005; Del Potro and Hürlimann 2007). The process of intrusion of dykes took place until the end of the Cycle I, a period in which the orientation of the stress field related to volcanism changed: from a radial pattern change to a NW-SE fissural regime (Yepes et al. 2011). In addition, the lateral spreading would be amplified by the age of the island, as it would provide a longer period to increase the weight of the volcano and, subsequently, to produce an unstable condition as other authors has been proposed in similar volcanic areas (Lo Giudice and Rasa 1992; Clague and Denlinger 1994; Van Wyk de Vries and Francis 1997; Borgia et al. 2000; Reid et al. 2001).

Fig. 3
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Shaded relief image showing the main geomorphological features observed at the NW sector of the Gran Canaria (see text for more details)

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The relief inflections allow distinguishing remnants of four different erosional surfaces (S0, S2, S4 and S6) (Figs. 2 and 3). The older structural surface (S0), defined by the lava flows of the Phonolitic Formation (Cycle I), can be related to a surface of summits where the lava flows of the subsequent volcanic stages (Roque Nublo, Post-Roque Nublo and Recent cycles) have been gradually embedded. Hence, the most recent surface (S6) defines the most embedded reliefs in the NW sector of the island. We have noticed a temporal alternation between these erosion surfaces and the stratigraphic unconformities (S1, S3 and S5), which are recognized later with lithological cross sections and interpreted as the sliding surfaces. This alternation of forms and processes support the hypothesis of large island flank instability along several volcanic cycles. This idea would be compatible with the existence of one or more translational rock slides with a very slow rate of movement. Triggering factors for this type of instability would be very old, reducing their energy. This fact suggests three remarks: (a) the recent processes of erosion could be able to generate new morphologies but not to remove the inherited forms, (b) the lava flows of the most recent cycles are embedded in the preexisting reliefs, and (c) the old sliding surfaces cover a greater extent than modern sliding surfaces.

The wide NE-SW valley is flanked by steep scarps, where old materials of the Cycle I outcrop (Figs. 3 and 4). These materials are difficult to correlate topographically with the surface of summits described above (S0) and, in addition, the southern scarp is covered by recent rock slides. These observations suggest that this valley could be the geomorphologic evidence of a secondary scarp (S5) that could be developed at the foot of a prior sliding mass (S3), resulting in a large rocky block (Montaña Almagro) disconnected of the main trend of the topography (Figs. 2 and 3). This hypothesis would be in agreement with the debris-avalanche deposits identified by other authors at the foot of the island shelf, between Agaete and Gáldar (Acosta et al. 2003).

Fig. 4
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Map of the superficial geology of the NW sector of Gran Canaria showing the main volcanic and erosional stages. The location of the five lithological cross-sections (black lines) and the 71 wells and boreholes (red dots) are also depicted

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Along the northern coast, there is a very steep escarpment with a height of several hundred meters. This relief has been interpreted as a result of a general tilt of the island to the south (Menéndez et al. 2008). However, the offshore investigations of the island shelf show the existence of several debris avalanche deposits close to Galdar (Acosta et al. 2003). Therefore, the complexity of the vertical movements experienced by the volcanic terrains indicates a polygenic character to the morphogenesis of the relief. The height of these coastal cliffs is compatible with the occurrence of an old flank slide and a later differential rising of a coastal block of the island.

Geological Investigations

Lithological cross-sections (Fig. 5) show sharp changes in the thickness of the formations of the Roque Nublo and Post-Roque Nublo cycles. The contact between the two formations suggests the existence of a paleo-relief compartmentalised in wide flat-bottomed valleys, separated by residual reliefs. This paleo-relief defines an erosion surface (S3) embedded between the remains of two previous structural surfaces: the top of the Roque Nublo Cycle (S2) and the surface of summits (S0).

Fig. 5
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Longitudinal (1–3) and transversal (4–5) cross-sections derived from the wells and boreholes data (red lines). See Fig. 4 for localization

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The age of the paleorelief is implied by a recent structural surface (S4) which is defined by the lava flows of the Post-Roque Nublo Cycle of Pleistocene age. A thinning of the prior substratum, defined by the most recent materials of the Cycle I (Trachytic-Rhyolitic and Phonolitic formations), has been found below the paleo-relief described above (S3). This discontinuity (S1) confirms the occurrence of previous translational rock slides of Miocene to Pliocene age.

Discussion and Conclusions

The large flank instability found in the northwest sector of the Gran Canaria Island would be related to the intrusion of dykes during the end of the Cycle I and suggests a NE-SW extensional regime, which could be related to a central NW-SE fault zone that divides the island in two sectors: northeast and southwest. This stress regime seems to be established for an extended period of time, suggesting the coexistence of other triggering factors such as the lateral spreading of the volcanic edifice due to the combined effect of the weight of the volcano and the time. Moreover, the existences of two different lahar deposits (Las Palmas Detritic Formation) in the stratigraphic sequence of the island (Balcells et al. 1992) suggest the occurrence of highly explosive eruptions. The earthquakes related to these explosive periods could also be considered as a triggering factor of the large flank instabilities in Gran Canaria Island.